This invention relates to spread-spectrum communications, and more particularly to a multipath processor, variable bandwidth device, and power control system.
Spread-spectrum modulation provides means for communicating in which a spread-spectrum signal occupies a bandwidth in excess of the minimum bandwidth necessary to send the same information. The band spread is accomplished by modulating an information-data signal with a chipping-sequence signal which is independent of an information-data signal. The information-data signal may come from a data device such as a computer, or an analog device which outputs an analog signal which has been digitized to an information-data signal, such as voice or video. The chipping-sequence signal is generated by a chip-code where the time duration, Tc, of each chip is substantially less than a data bit or data symbol. A synchronized reception of the information-data signal with the chipping-sequence signal at a receiver is used for despreading the spread-spectrum signal and subsequent recovery of data from the spread-spectrum signal.
Spread-spectrum modulation offers many advantages as a communications system for an office or urban environment. These advantages include reducing intentional and unintentional interference, combating multipath problems, and providing multiple access to a communications system shared by multiple users. Commercially, these applications include, but are not limited to, local area networks for computers and personal communications networks for telephone, as well as other data applications.
A cellular communications network, using spread-spectrum modulation for communicating between a base station and a multiplicity of users, requires control of the power level of a particular mobile user station. Within a particular cell, a mobile station near the base station of the cell may be required to transmit with a power level less than that required when the mobile station is near an outer perimeter of the cell. This adjustment in power level is done to ensure a constant power level is received at the base station from each mobile station.
In a first geographical region, such as an urban environment, the cellular architecture may have small cells in which the respective base stations are close to each other, requiring a low power level from each mobile user. In a second geographical region, such as a rural environment, the cellular architecture may have large cells in which the respective base stations are spread apart, requiring a relatively high power level from each mobile user. A mobile user who moves from the first geographical region to the second geographical region typically adjusts the power level of his transmitter in order to meet the requirements of a particular geographic region. If such adjustments were not made, a mobile user traveling from a sparsely populated region with larger cells, using the relatively higher power level with his spread-spectrum transmitter, to a densely populated region with many small cells may, without reducing the original power level of his spread-spectrum transmitter, cause undesirable interference within the smaller cell into which he has traveled and/or to adjacent cells. Also, if a mobile user moves behind a building and has his signal to the base station blocked by the building, then the mobile user's power level should be increased. These adjustments must be made quickly, with high dynamic range and in a manner to ensure an almost constant received power level with low root mean square error and peak deviations from the constant level.
Accordingly, there is a need to have a spread-spectrum system and method for automatically controlling a mobile user's spread-spectrum transmitter power level when operating in a cellular communications network.